The rkp-3 gene region of Sinorhizobium meliloti Rm41 contains strain-specific genes that determine K antigen structure

The rkp-3 region is indispensable for capsular polysaccharide (K antigen) s ynthesis in Sinorhizobium meliloti Rm41. Strain Rm41 produces a K antigen o f strain-specific structure, designated as the K(R)5 antigen. The data in t his report show that the rkp-3 gene region comprises 10 open reading frames involved in bacterial polysaccharide synthesis and export. The predicted a mino acid sequences for the rkpl-Q gene products are homologous to enzymes involved in the production of specific sugar moieties, while the putative p roducts of the rkpRST genes show a high degree of similarity to proteins re quired for transporting polysaccharides to the cell surface. Southern analy sis experiments using gene-specific probes suggest that genes involved in t he synthesis of the precursor sugars are unique in strain Rm41, whereas seq uences coding for export proteins are widely distributed among Sinorhizobiu m species. Mutations in the rkpL-Q genes result in a modified K antigen pat tern and impaired symbiotic capabilities. On this basis, we suggest that th ese genes are required for the production of the K(R)5 antigen that is nece ssary for S. meliloti Rm41 exoB (AK631)-alfalfa (Medicago saliva) symbiosis .